We
now proceed to examine some nonclassical properties of the
nonlinear coherent states \(|\beta \rangle _f\) . As an important
quantity, we consider the variance of the number operator
\(\hat{n}\) . Since for the coherent states the variance of number
operator is equal to its average, deviation from the Poissonian
statistics can be measured with the Mandel parameter [1]}
\(M=\frac{(\Delta n)^2-\langle \hat{n}\rangle }{\langle \hat{n}\rangle }.\) 
